Vortical flow aerothermodynamic fireplace unit

ABSTRACT

A heat exchanger particularly for use in a circulating hot air fireplace is constructed to induce vorticity in the flame and/or hot combustion gases at a position in proximity to a heat exchange surface to enhance residence time and thermal transfer to the circulating air. 
     The vorticity pattern acts as a self-adjusting flow controller.

FIELD OF INVENTION

This invention relates generally to .Iadd.heat exchangers, and isparticularly applicable to furnaces and .Iaddend.fireplace units whereinroom air is circulated, either by convection or by mechanical forcingmeans, in heat exchange relationship to a combustion chamber andreturned to the room in heated condition.

BACKGROUND OF THE INVENTION--PRIOR ART

Considering first the prior art relating to air circulating fireplaces,it is known to construct fireplaces or inserts therefor which providemeans to circulate room air through passages in the walls defining thecombustion chamber to absorb heat from the source, after which theheated air is returned to the room. This art includes elaboratelabyrinthian passages for the room air and combustion air alike in anattempt to lengthen the period of residence of the respective flows inmutual heat exchange relationship, as exemplified by U.S. Pat. No.2,642,859, issued June 23, 1953 to Newman T. Brown. Moreover, it hasbeen proposed to so dimension the combustion chamber that an unconfinedslowly descending recirculating flow of combustion air is encouraged, asseen in U.S. Pat. No. 773,863, issued Nov. 1, 1804 to Mary F.Frecktling, and to provide confined passages to direct a recirculatingflow, as in U.S. Pat. No. 2,821,975, issued Feb. 4, 1958 to Robert K.Thulman, in U.S. Pat. No. 2,185,788, issued Jan. 2, 1940 to August R.Fredlund, and in U.S. Pat. No. 53,880, issued Apr. 10, 1866 to FrancisM. Rogers. It is noted that the Frecktling disclosure recirculates onlythe slowly moving portion of the combustion products, the principle heatcontaining portion passing directly to the flue. On the other hand, theother disclosures in which substantial portions of the combustion floware recirculated in confined paths requires the introduction ofstructural impedance to the gas flow and depends upon the presence of alarge expanse of heat exchange surface.

Turning to the art relating to combustion generally, it is well known toinduce a helical flow of a fuel/air mixture in order to increase theresidence time of the mixture within the combustion zone and thusenhance complete combustion, and it has been suggested that such aneffort may be augmented by restricting the outlet of the combustionchamber or by introducing a supplemental forced air flow. For an exampleof this art, reference is to U.S. Pat. No. 3,007,310, issued Nov. 7,1961 to Karl Eisele and to U.S. Pat. No. 3,258,052 issued June 28, 1966to Alfred Wilson, et al. Augmentation of the spiral flow of air/fuelmixture has also been proposed by flow conditions which induce anannular tore comprising a flame vortex adjacent the base of the flame inU.S. Pat. No. 3,030,773, issued Apr. 24, 1962 to Robert H. Johnson andin U.S. Pat. No. 3,255,802 issued June 14, 1966 to James A. Browning. Asimilar flow is induced within the area of air/fuel mixing in U.S. Pat.No. 3,118,489, issued Jan. 21, 1964 to Clifford C. Anthes.

Considering the prior art even more generally, in the field of heatexchange it is known to induce a gaseous medium to flow in a vorticalpattern extending axially of a tubular conduit in order to increaseresidence time, enhance scrubbing action and to obtain an interchange ofposition of the molecules of high velocity and temperature gases fromthe center of the vortex with the outer molecules which have beendeprived of their energy and velocity through functional heat exchangecontact with the vortex tube in which the vortical flow is confined.This is exemplified by the well known "Ranque" tube (U.S. Pat. No.1,952,281, Mar. 27, 1934) and see U.S. Pat. No. 2,586,002, issued Feb.19, 1952 to W. R. Carson, Jr. et al.

In summary, the prior art is known to disclose inducement of vorticalflow in precombustion gases and basal portions of flame patterns for thepurpose of enhancing the mixing of the fuel air mixture, and the priorart discloses inducement of hot gas vorticity axially of confinedconduits of heat exchangers.

OBJECTS OF INVENTION

In contradistinction of the foregoing, it is among the objects of thisinvention to provide an aerothermodynamic heat exchange structureincluding features which

1. a stable, relatively unconfined flow of hot gases is induced andmaintained throughout varying conditions of temperature and velocity,

2. the hot gas vorticity is established proximate to a heat exchangesurface at which hot gas residence time is prolonged, thus enhancingheat exchange,

3. the vortex axis of the hot gas is perpendicular to the hot gas flowentering and exiting the vortex area whereby the vortex fluid impedancevaries with variations in hot gas velocity,

4. a vortical hot gas flow pattern is maintained in a heat exchangerwhich presents minimal structural impedance to gas flow,

5. a hot gas flow path is maintained free of areas of aerodynamicstagnation,

6. the area of heat exchange surface is minimized,

7. the area of frictional contact of flowing gases with flow conduits ismaximized,

8. a self-regulating draft is established by vortex imposed aerodynamicimpedance,

9. the structure is readily adaptable to domestic furnace or fireplaceinstallation as original equipment or as a modification of preexistingconventional fireplaces or furnaces.

DESCRIPTION OF DRAWINGS

The aforestated objects, as well as other objects inherent in theapparatus of this invention will be apparent from a consideration of theensueing specification and reference to the drawings, in which

FIG. 1 is a perspective view of a fireplace unit having portions of thefront end and one side broken away to reveal interior features in crosssection,

FIG. 2 is an elevational cross section taken through line 2-- 2 of FIG.1,

FIG. 3 is a view similar to that of FIG. 2 of an alternative embodiment,

FIG. 4 is a dimensional diagram of a preferred embodiment,

FIG. 5 sets forth alternative duct configurations,

FIGS. 6, 7 and 8 are perspective view of a portion of a fireplace,

FIG. 9 is a perspective view of a portion of a fireplace broken away todisclose door mounting details,

FIG. 10 is a plan view of the door in closed position, and

FIG. 11 is a horizontal cross-section, a portion of a fireplacerevealing a door in stored position.

TERMINOLOGY

This invention relates to the phenomena of heat exchange between aheated high velocity gas induced to flow in a relatively confinedvortical pattern in close proximity to a heat exchange surface throughwhich heat is transmitted to a relatively cooler fluid. In order tomaintain a distinction between the aforementioned prior art in whichvortical patterns are produced at basal portions of a flame for thepurpose of enhancing combustion, this specification will refer to theheated gas as the donative gas and to the cooler fluid as the recipient.Thus, donative gas is that gas which has been brought to a temperaturecondition where it is ready to be introduced into the heat exchangerelationship and may include portions of the flame in which combustionis sufficiently complete to have brought about the aforesaid temperaturecondition, as well as combustion products immediately downstream of aflame, or gas heated at a remote point. Recipient fluid, on the otherhand, is any fluid, i.e., liquid or gas, which received heat from thedonative gas.

DESCRIPTION OF INVENTION

Referring first to FIGS. 1-3, there are depicted fireplace units, eachcomprising an outer enclosure generally designated 1 and including a topwall 2, side walls 3 and 4, and a back wall 5. Spaced inwardly from saidouter enclosure walls is a fire enclosure defined by a top wall 6, sidewalls 7, 8, a back wall 9, and a bottom 10. Both enclosures share acommon partial front wall 11 extending downwardly from the top wall 2and defining a plurality of openings, namely an upper recipient gas exitat 12, a lower recipient gas exit at 13, and a fire enclosure opening at14. A barrier lip 15 on the front wall which is immediately superjacentto the fire enclosure bottom 10 for purposes to be elaborated on in theensueing specification, includes one or more openings 16 to provide theentrance of combustion air. These openings may be provided withappropriate flow control valves (not shown) to provide controllabledraft. A combustion gas exhaust passage for communicating the fireenclosure with a flue is defined by a duct 17. FIG. 3 disclosesalternative embodiment configured so as to be particularly adaptable toexisting fireplaces, and wherein the backs 5, 9 and top 2 are sloped.

A still further alternative (not shown) would eliminate the outerenclosure and utilize the existing fireplace enclosure in lieu thereof.

In FIGS. 1-3 the path of the room air through the unit, wherein it istermed recipient air to denote its function of reception of heat forconveyance to the room area by convection, is traced by dashed linearrows, whereas the path of heated combustion products, termed donativegas, is denoted by solid line arrows. In the latter regard, particularattention is invited to the path of the donative gases (which mayinclude the flame under some conditions and/or the intensely heatedgases downstream of the flame under other conditions) by which they arebrought in contact with the undersurface 18 of a duct 19 interconnectingthe recipient air passage through an opening in rear combustion chamberwall 9 with the recipient duct exit 13. The undersurface forms a flameplate whereby, at the points of juncture of this flame plate duct 19with the partial front wall and with the rear wall, the donative gasflow is induced to flow in a pattern of vorticity which remains stablethroughout a wide range of temperature and velocity. Again, afterleaving the aforementioned vorticies, the combustion gases encounter thejuncture of the back and front walls 9 and 11 with the flame enclosuretop wall 6, vortical flow patterns are again established and maintainedin a stable persistent pattern throughout varying flow conditions. Inthe present model, four front vortices exist under all tested operatingconditions and four additional (rear) vortices arise when the fireextends sufficiently rearwardly in the flame enclosure. Additionalvortices may be induced by the provision of lateral fins 20 (FIG. 3) tothe underside of the flame plate surface 18. These fins serve to augmentthe stabilization of the aforementioned vortices and to establishadditional vortices either independently under high velocity conditions,or as the original stable vortex increases in translational velocity andin circumference to a point where it overflows the partial barrierformed by the fin and adopts a vortical flow pattern in the adjacentchannel defined by the flame plate 18 and the fins 20.

While depicted in FIGS. 1-3 as a complete insert unit, it is readilyapparent that the essence of this invention is equally applicable to aninsert which utilizes an existing fireplace as the flame enclosure, andwherein the insert includes only the flow diverting flame plate, and thevortex defining juncture is formed of additional flow diverting elementsextending from a juncture with the flame plate toward the flame.

In each instance, the stable vortical patterns of donative gas flow areestablished by structure which presents a partial barrier (i.e. theunderside 18 of the flame plate duct 19 and the top of the flameenclosure) to the otherwise free unobstructed flow of hot donative gastoward the flue while permitting the flue to draw off gases from the endof the vortex so formed. The latter function is enhanced by slanting thebarrier in its longitudinal direction upwardly in the direction of gasflow so that the long axis of the vortex coincides with the predominentdirection of ultimate gas flow and thus leads toward the flue opening.

In addition to the aforementioned longitudinal slant of the flame plate19, it is desirable to provide a pitch in a direction transverse to theslant direction, thus to establish an axial flow within the vorticitypattern. To this end, the flame plate 19 should have a transverse pitchof approximately 15 degrees, the pitch being transverse to the slant anddirected upwardly to a free edge which edge coacts with a contiguousarea of the enclosure wall 8 to define a portion of the donative gasflow path therebetween. In the preferred embodiment wherein the flameplate 19 extends through the center of the enclosure, thus dividing theenclosure into two donative gas flow portions, the flame plate divergeslaterally outwardly from a longitudinal central portion of the plate toterminate at two free edges defining the two flow path portions.Examples of permissible cross-sectional configurations of a flame plateduct of the last-named type are diagramatically illustrated in FIG. 5.These include a triangle (FIG. 5A), a modified triangle or polygon (FIG.5B), a chevron (FIG. 5C), and an ellipse (FIG. 5D).

A preferred size as set forth in relation to FIG. 4 prescribes ahorizontal minor dimension of 22 inches, a vertical minor dimension of 3inches, and a divergent pitch of 15° in the flame plate surface. Thispitch is established at 0.5 to 2 times the slant angle of flame plateduct 19 from the rear wall 9 to the partial front wall 11. The aim is topresent a partial barrier to the upward donative gas flow, thus causingthe gas to arrive at the aforementioned junctures and form vorticescommencing at the low point (center portion) of flame plate 19 andextending upwardly in each lateral direction to terminate at a free edgeof said flame plate spaced from a contiguous portion of the enclosurewall 8.

The cross sectional pitch configuration and longitudinal slant of theduct also has certain beneficial effects on the flow therethrough ofrecipient gases. First, the combination of upward pitch from front torear and from center to sides tends to encourage lateral flow patternswithin the flame plate duct 19 by virtue of the increased tendency ofthe heated recipient gas to lift off a sloped surface. Thus, slowlyspiralling counterrotating recipient gas currents occuring at respectivesides of the center line of the duct disrupt otherwise lamellar flowpatterns, whereby to assist in the suspectibility of the recipient gasto heat exchange. Secondly, the preferred embodiment of FIG. 5B enhancesthis circulation by providing sufficient heights above the extremelateral extent of the flame plate 18 for the lift off of recipient gasto occur, tending toward an equilization of flow through the duct 19 atits center and at its sides. The sides 21 should be limited in height asshown inasmuch as they are not in proximity to the vortices of thedonative gas and hence are relatively ineffective as heat transfersurfaces.

The sum of the areas of the two recipient gas exits 12 and 13 withrespect to the area of the bottom air inlet should be such that the massflow of the inlet gas at room temperature approximates the sum of themass flows of the recipient gas at the respective temperatures of exit,which have been found typically to be 200° F. at exit 12 and 300° F. atexit 13.

In a model dimensional as set forth in connection with FIG. 4, stablevortices are maintained at rotational speeds of approximately 100rotations/second in a vortex of 4 inches diameter while the longitudinalvelocity is about 2 feet/second. Since, in this example, heat transfersurface is present around half of that circumference, an effectiveexchange surface path over a one foot segment of the distance from fireto flue is ##EQU1## Where the heat exchange surface surrounds more thanhalf of the vortex, the denominator is correspondingly decreased. Sinceeach vortex is 2 feet long, the effective scrubbing or heat exchangesurface is 50× 2=100 square feet, whereas the volume of each vortex is##EQU2##

The vortices tend to decrease the effective size of the donor gasconduit to the flue by expanding into the open area of the flue, orstated conversely, tend to increase the aerodynamic impedance. Empiricaldesign can achieve a self adjusting system wherein the effective flueimpedance is least when the convection is least and increases asconvection increases. This mechanism is not fully understood, butappears to depend upon the matching of aerodynamic impedance at variouscross sections along the flow path in such a way that the vorticesenlarge and contract in diameter as the fire increases and decreases inheat output. To this end, the vortex inducing structure must not totallyconfine the circumference of the vortex, but must leave an opening on atleast one side sufficient to permit the aforementioned expansion. Anincrease in diameter of adjacent vortices brings about an increasedchoking effect to the straight flow of gases therebetween, thus servingas a damper to increase aerodynamic impedance as fire intensityincreases. The design should preserve a consistency of aerodynamicimpedance throughout the flow path of the donor gas, i.e., the sum ofthe cross-sectional open air areas in the upper reaches of thecombustion chamber should approximately equal the cross-sectional areaof the flue duct 17, which is smaller than the average chimney flue incross-section. Moreover the partial barrier formed by the undersurface18 of the duct 19 must be substantial and has been found to be mosteffective when the width of the duct approximates 1/4 or more or thetotal width of the flame enclosure. Adherence to these basic parametershas found embodiment in an experimental fireplace including a fireenclosure dimensioned as indicated in FIG. 4 having a width of 32 inchesin the front tapering to a 28 inch width in the rear, a front height of29 inches, and an 18° slope from front to rear of the enclosure top 2and of the recipient air duct 19. This unit includes a 4 inch barrier 15and has a front-to-rear depth of 21 inches at the base, tapering to a 15inch depth at the top. An optimum flue duct opening 17 is 48 squareinches, and is preferably symmetrically triangular or polygonal in shapewith a major dimension extending to an apex in the rear half of the top6 (see FIG. 1). Inasmuch as the slant angle of the flame plate 18 servesto direct the major donative gas flow toward the juncture with thepartial front wall 11, the one front juncture is the first to form avortex and is the preferred heat exchange area. Hence, it is desireableto position the flue duct so as to induce the major donative gas flow inthe front of the enclosure. A triangle having an 8 inch base positionedapproximately 4 inches behind the forward edge of top 6, and said majordimension of 12 inches maintains a 48 square inch cross sectional areaand positions the apex near the rear of the top 6. Flue duct openings aslarge as 64 square inches are feasible, as are variations in crosssectional configurations, such as square, rectangular, trapezoidal,parallegram. In any configuration, however, the major area of theopening should reside in the front half of the top 6 where it isdownstream of said one front juncture.

Another design factor which has a surprisingly significant effect on theoverall balance of aerodynamic impedance is the impedance presented bythe barrier lip 15. Surprisingly, the width and height of the openingabove the lip 15 have little effect, yet a barrier lip which is too lowresults in instable vortices and inefficient drafts. It appears that thepredominant flow of combustion air into the flame chamber is through thelower portion of the opening superjacent to the lip 15. Hence, theeffect of the height of this barrier lip relative to the position of thefire is significant due to imput damper effect on the predominant flow.

In view of the pronounced effect of the impedance offered by therelative height of barrier lip 15 in relation to the fire, it isproposed in an additional embodiment to provide an inbuilt gratestructure whereby the fire will be supported at a predetermined heightwhich is not dependent upon that of an independently acquired grate.Such a structure is illustrated in FIG. 6, wherein a pair of spacedgrates 21 extend from front to rear. The grates have a cross-sectionalconfiguration of an inverted channel with the underside open through theflame enclosure bottom 10 to permit the flow of recipient gas into andthrough the channel shaped grates 21. This opening may extend the entirelength of the grate, or may involve only a portion thereof. It ispreferred that the channel so provided open through the fire enclosuureback 9 to provide unobstructed flow of the recipient gas to thepassageway defined between surfaces 5 and 9. This hollow grate thusprovides additional heat exchange surface in close proximity to the firewhereby heating of the recipient gas is augmented and, conversely, theflow of relatively cool incoming recipient gas has a cooling effect onthe hollow grate which avoids the danger of burnout of the metalstructure.

Still further in recognition of the significant effect of the height ofthe barrier 15 relative to the fire, this invention contemplates abarrier of adjustable height, which may take the form but is exemplifiedby a pivoted vane 22 of FIG. 7 positioned between the grates 21 asshown, or may be positioned on the upper edge of barrier 15, in whichinstance the vane 22 can extend the full width of the fire enclosureopening. Either structure may be manually controlled, as by a handle 23attached to the vane pintle 24, or it may be thermostataicallycontrolled by a motor 29 responsive to a thermosensitive element (notshown) placed in the room environment. As the barrier vane 22 is raised,the upper limit of fire intensity is decreased.

In each of the aforedescribed embodiments, dependence is solely uponnatural draft. A somewhat faster draft control may be effected by aforced draft created by a fan, such as a "muffin" fan 25 placed in thechannel of the grate 21 as seen in FIG. 8. This modification isparticularly useful during start-up inasmuch as the fan 25 may be usedto forcibly supply ambient air to the recipient air passageways as inthe previously described embodiments while at the same time bleeding offa portion of the forced air flow as combustion air to the fire. To thisend, a draft opening 26 is provided on the fire side of grate 21, andsaid opening is controlled by a vane 27 shown as being manuallycontrolled as by handle 28. Vane 27 may, of course, be controlled by athermally sensitive system, as by a bimetallic motor element (not shown)responsive to a thermal sensor which appropriately may be placed in therecipient air duct discharge opening 13. The use of the same forced airsupply for recipient air and for combustion air is particularlyadvantageous as the optimum mass flows of the two bears an inverserelationship. At start-up or other low burning conditions during whichheat exchange surface temperatures may be as low as 100°-200° F., theratio of mass flow of recipient air to draft air should be relativelylow, whereas during high intensity burning conditions at temperaturesof, say, 800°-1000° F., the opposite is true. The fan 25 may becontrolled by a room thermostat of conventional design (not shown). Fan25 advantageously is placed near the room air intake where convectiveflow of the ambient temperature air is sufficient to cool the fan whennot operating and thus to avoid heat damage thereto.

A further advantage of the basic structure of this invention resides inthe capacity of the unit to function with no loss of efficiency whenglass doors are employed in the fire enclosure opening 14. Glass doorshave been employed in conventional fireplaces for purposes of safety andof eliminating excessive loss of room air to the flue. However, thedoors accomplish these objectives only at a sacrifice of heating valueto the room by virtue of the elimination of convective flow fromenclosure to room and diminished direct radiation. In this invention, inwhich major dependence is upon heat exchange from donative gas to therecipient gas and subsequent convective or forced air flow to the roomrather than upon direct convection and radiation, the sacrifice isminimized to an extent of virtual inconsequence while the advantagesstill attain.

To this end, there may be provided as seen in FIGS. 9-11, a recess inone or both sides of the fireplace, the recess being partially definedby side walls 4 and 8. Disposed within said recess is a track comprisingan upper element 30 and a lower element 31 secured in place by anysuitable means, such as brackets as at 32. Slidable in the track 30, 31is a carrier generally indicated at 31 which carries a hinge pintle 34.Pivotally supported upon pintle 34 is a door here shown as a foldingdoor comprising a pair of glass panels 35, 36 pivotally interconnectedas a hinge pintle 37. A nonfolding door can be provided, in which itwould be appropriate to provide a track and carrier on each side of thefireplace to accommodate duplicate doors. In the preferred embodiment ofa folding door, a convenient handle 38 is attached to the hingedlyconnected end of door 36 comprised a free end which is forked to providea hooked finger tab 39 and a straight thumb tab 40. By virtue of itsplacement at a low position on the door, this handle remains cool to thetouch. By engagement of the thumb and forefinger with the tabs 40 and39, respectively, the door may readily be pulled from its retractedposition shown in FIG. 11 to be extended position of FIG. 9, this motionbeing accommodated by the sliding of the carrier 33 in the trackelements 30, 31. At this point, a rotary motion by the thumb and fingerpivots door 36 with respect to door 35 and permits bringing the door tothe closed position illustrated in FIG. 10, at which a permanent magnet41 retains the door in position against barrier 15.

The fireplace of this invention is particularly well adapted to the useof doors such as the aforedescribed partially because the reliance onunderdoor combustive air intakes makes the unit independent of thenormal fire enclosure opening for adequate combustion. This isparticularly true with the forced air provision of FIG. 8, and to alesser extent in units involving natural draft intakes such as 16.Another factor in adaptability to doors is the reliance on convectiveflow of the recipient gas rather than dependence upon direct radiationto the room. Finally, the ambient air passageway defined in part bysidewalls 4 and 8 provides a convenient receptacle for the doors whenopen and in stored condition, the interference of the door withrecipient gas flow being insignificant, whereas the radiation reflectiveproperties of the glass provide supplemental insulation to reduce heatloss through outer side wall 4.

While described in the foregoing specification in the preferredembodiment of a fireplace, the aerothermodynamic heat exchanger of thisinvention which employs vortical flow patterns to enhance heat transferbetween a relatively unconfined donative gas flow path and an isolatedrecipient gas flow may find numerous other heat exchange applications.Hence, the scope of this invention is not considered to be limited bythis specification, but should be construed in accordance with thefollowing claims.

I claim:
 1. A heat exchange structure for use .[.in a fireplace.]..Iadd.with a firebox .Iaddend.of the type including an enclosure for afire producing a hot thermally donative gas, said enclosure comprisingside walls, a back wall, and a top wall,a flue opening in said top wallfor exhausting said donative gas, an outer enclosure comprising wallsspaced outwardly from said fire enclosure walls and definingtherebetween passages for a thermally recipient air, a front wallextending downwardly from a front portion of said top wall and includinga fire enclosure opening therein, means communicating air from saidthermally recipient air passages through said front wall, said meanscomprising a duct extending between junctures with said back and frontwalls through a central portion of said fire enclosure, said structurefurther characterized in that said duct has a bottom surface comprisinga flame plate for exposure to said donative gas in said enclosure, saidflame plate being slanted upwardly toward one of said junctures todefine throughout the extent of said one juncture a laterally extensivethermally donative air vorticity area throughout which the donative airis induced to whirl in a stable vortex proximate to and in heat exchangerelationship to said flame plate and said wall structure.
 2. Thestructure of claim 1 wherein said flame plate includes a free edgespaced from a contiguous portion of said fire enclosure side wall todefine a portion of said donative gas flow path therebetween, said flameplate being pitched in a direction transverse to said slant and towardsaid free edge whereby the axis of the induced vortical pattern ispitched in the general direction of gas flow and terminated beyond saidfree edge.
 3. The structure of claim 2 wherein the cross-sectional areaof said portion of said donative gas flow defined by said free edge andsaid contiguous enclosure wall portion bears a substantially equalrelationship to the cross-sectional area of said exhaust flue opening.4. The structure of claim 1 wherein said flame plate includes two freeedges each spaced from the respective contiguous portions of said fireenclosure walls to define on either side of said duct a pair of portionsof said donative gas flow paths therebetween, said pitch of said flameplate diverging laterally outwardly from a longitudinal central portionthereof to each said free edge.
 5. The structure of claim 4 wherein thesum of the cross-sectional areas of said donative gas flow paths oneither side of said duct bears a substantially equal relationship to thecross-sectional area of said exhaust flue opening.
 6. The structure ofclaim 4 wherein the width of same flame plate is at least 1/4 the widthof said flame enclosure.
 7. The structure of claim 4 wherein said pitchis substantially 15°.
 8. The structure of claim 7 wherein the ratio ofsaid slant angle to said pitch is substantially within the range of 0.5to
 2. 9. The structure of claim 1 wherein said exhaust opening has itsmajor area disposed downstream of said one juncture.
 10. The structureof claim 9 wherein said exhaust flue opening has at least a portionthereof of substantially triangular cross-sectional configuration, thebase of said triangular constituting said major area and the apexextending away from said one juncture toward the other of said juncturesdownstream thereof.
 11. The structure of claim 1 wherein said flameplate includes lateral fins projecting downwardly into said fireenclosure and extending substantially parallel to and spaced from saidjunctures, said fins coacting with said flame plate surface to defineadditional areas for vorticity patterns of donative gas flow.
 12. Thestructure of claim 1 wherein said front wall includes an additionalportion comprising a barrier lip extending upwardly from said fireenclosure bottom, said barrier lip being adjustable in height.
 13. Thestructure of claim 1 wherein said fire enclosure bottom includes spacedelongated hollow grates extending from front to rear, said gratescomprising inverted channel shaped elements having at least a portion ofthe underside of said channel open through said bottom and being open tocommunicate with said passage for thermally recipient air.
 14. Thestructure of claim 13 wherein said tubular grate elements include adraft opening in a side of said tubular element for communicating aportion of the accepted ambient room air with the lower portion of saidfire enclosure.
 15. The structure of claim 14 wherein said draft openingincludes a flow control vane, said vane being controlled by a thermallysensitive element.
 16. The structure of claim 1 wherein said front wallincludes an additional portion comprising a barrier lip extendingupwardly from said fire enclosure bottom, and wherein said fireenclosure includes spaced grate elements extending from front to rear,said grate elements each comprising a tubular member open through saidbarrier lip to accept ambient room air and open through said enclosurerear wall to discharge air to the rear said passage for thermallyrecipient air.
 17. The structure of claim 16 wherein said tubular grateelements include air forcing means disposed within said tubular elementnear said opening through said barrier lip.
 18. The structure of claim 1wherein said front wall has a recess adjacent to said fire enclosureopening, said structure including a door assembly movable from a closedposition covering said flame opening to a stored position within saidadditional opening.
 19. The structure of claim 18 wherein said recess isdisposed within said recipient air passage, a horizontal track isdisposed within said recess, a carrier is slidably mounted upon saidtrack, and said door assembly is pivotally mounted to and carried bysaid carriage for pivotal movement from said closed position to an openposition aligned with said recess, and said carrier is movable from anextended to a retracted position within said recess to totally confinesaid door assembly within said recess.
 20. The structure of claim 9wherein said door assembly comprises a folding door composed of at leasttwo panels, one said panel having one end pivotally supported by saidcarrier and another said panel being hingedly connected to theunsupported end of said one panel, a handle element rigidly secured tothe hingedly connected end of said another panel whereby said handle maybe used for movement of said door assembly between said closed, open andstored positions.
 21. A heat exchange structure for use .[.in afireplace.]. .Iadd.with a firebox .Iaddend.of the type including.Iadd.an.Iaddend.enclosure for a fire producing hot, thermally donative gas, aflue opening for exhausting said gas, means for conducting ambient roomair in heat recipient relationship to said hot, thermally donative gas,said structure characterized in the inclusion of means to divert atleast a portion of said thermally donative gas in a vortical pattern,said means comprising a duct comprising a portion of said recipient gasconducting means, said duct having a bottom surface comprising a flameplate exposed to said donative gas in said enclosure to form a partialbarrier to the flow of said donative gas to said flue opening,additional partial barrier means comprising a surface extending towardsaid donative gas source and intersecting said bottom surface to formtherewith a juncture defining a laterally extensive thermally donativeair vorticity area throughout which donative air is induced to whirl ina stable vortex proximate to and in heat exchange relationship to saidflame plate.